1. Field of the Invention
This invention relates to electronic systems. Specifically, the present invention relates to electronic test systems.
2. Description of the Related Art
Modern electronic systems are sophisticated and complex. As a result, supporting technology such as testing technology has increased in complexity. Electronic systems that once included hundreds or thousands of circuits now include millions of circuits. As a result, more sophisticated electronic testers are being deployed to test these electronic systems.
Modern electronic systems are often deployed in printed circuit boards (PCB's) or in integrated circuits with multiple analog and discrete components. Both the printed circuit boards and the integrated circuits include millions of devices (e.g. transistors, logic gates, etc). Since printed circuit boards and integrated circuits are relatively small in dimension, these devices are deployed within a small area. As a result, extremely sophisticated electronic testers are deployed capable of testing thousands of devices within a small area.
A conventional PCB has a number of layers that perform various functions. The layers may be made of a variety of materials, including conductive materials and dielectric materials depending on the function of the PCB. A conventional PCB board also includes thousands of connections between devices. The connections between devices are often referred to as traces. The traces serve as conduits for carrying electrical current between the devices. The traces are made of a conducting material such as copper. Vias communicate signals from trace to trace between the various layers. Areas of the conducting material, known as pads, are formed on both the topside of the PCB and the underside of the PCB. The pads are connected by the traces and function as mounting locations for the devices and as a point of contact for testing the PCB.
A conventional electronic test system typically includes a tester and a fixture. The fixture acts as an interface between the tester and the device under test (DUT) or unit under test (UUT). The fixture primarily stabilizes the DUT and routes test signals from the tester to the DUT. The tester interfaces with the DUT through the fixture and applies voltages and/or current through the fixture to different points on the DUT. During a test, the tester measures the trace current or a voltage to determine the quality of a signal path or the operational characteristics of a device. The tester typically uses software to control and automate the testing process.
Conventional test systems include wired and wireless fixtures. Fixtures include a probe plate for holding a plurality of probes (e.g. known as a probe pattern or a probe field). The probes provide an electrical pathway from the tester to the DUT. The probes are held in place by the probe plate, which is positioned between the DUT and the tester. The probes make contact with the underside of the DUT on pads. Oppositely disposed ends of the probes are positioned to make contact with the tester through wiring or a PCB in the fixture.
The tester includes a uniform pattern of tester contact points. The tester contact points provide an electrical pathway for test signals generated from the tester. For example, in a conventional tester, the tester contact points are nails with serrated heads. The tester contact points are connected to the internal electronics in the tester. A conventional fixture such as a wired fixture is in contact with the tester contact points. An electrical pathway is established between the wired fixture and the tester contact points through pins known as personality pins. The personality pins are single-ended wire-wrap pins that are mounted in the bottom of a probe plate in the wired fixture. The personality pins make contact with the tester contact points and provide a connection point (e.g. wire-wrap tail), to make a wire connection inside the fixture.
In a wired fixture, the personality pins and the probes are located within proximity of each other in the fixture. Wires are connected between the personality pins and the probes. The wires are wrapped around the personality pins at one end and wrapped around the tester contact points at the other end. As a result, an electrical pathway for testing is established between the tester and the DUT. The electrical pathway begins at the internal electronics of the tester, runs through the tester contact points, through the personality pins and across the wire connection to the probes, which make contact with pads on the underside of the DUT.
As the numbers of devices on PCB's have increased and the sizes of the PCB's have decreased, it has become difficult to position probes and connect wires within a fixture. For example, it may be necessary to position probes on a DUT that has more than 5000 test locations. As a result, more than 5000 wires may need to be wire-wrapped in the fixture to establish the electrical connection between the tester and the DUT. This massive amount of wire results in an incredible amount of congestion in a very small area. In addition if there is a malfunction or mis-wired connection, it is very difficult to identify a dysfunctional wire. Therefore troubleshooting becomes a problem.
As a result, a more modern fixture assembly evolved which attempts to eliminate the need for wires in a fixture. This more recent version of the fixture is often referred to as a wireless fixture. In the more recent version, a fixture houses probes, which are used to engage pads on the underside of a DUT. A fixture PCB board or wireless PCB is positioned within the fixture and located on an oppositely disposed end of the probes. The wireless PCB includes a plurality of trace patterns for conducting electrical signals within the PCB between pads on both the topside and underside of the wireless PCB. Contact is made between the tester and the underside of the wireless PCB. As a result, an electrical pathway is established between the tester and the wireless PCB. The test signals are routed through the various trace patterns within the wireless PCB. Probes then make contact with the topside of the wireless PCB and an electrical pathway is established between the wireless PCB and the DUT. Ultimately, using the wireless PCB, an electrical pathway is established from the tester, through the wireless PCB, to the DUT.
In order to establish a good electrical pathway, the components of the fixture such as the wireless PCB must be positioned accurately. In a conventional wireless fixture, the wireless PCB is screwed down to hold the wireless PCB in place and support the PCB over the surface of the board. Stabilizing the wireless PCB by using screws to connect the wireless PCB to the fixture, introduces a significant number of problems. For example, there are initial fixture design and assembly problems.
The screws are placed so that they avoid the probe pattern and then traces are routed in the wireless PCB to avoid the screws. In addition, the screws are placed into the wireless PCB and fastened in a coordinated manner so that unbalanced forces don't appear in the Wireless PCB. Unbalanced forces may introduce fractures in the PCB, cause signal path breaks, or create misalignments; therefore, every attempt is made to avoid these unbalanced forces. For example, the screws are placed in the wireless PCB in a specific order and are tightened a quarter turn at a time.
In addition, placing screws into the wireless PCB introduces a tremendous amount of processing overhead. Screws are typically required where the probe density is very high to stabilize the wireless PCB. However, a high density of traces is also required, where the probe density is high. As a result, it becomes very hard to place the screws, when you have a high concentration of traces that need to be routed in and around a highly concentrated area of probes. The competing interest of probe density versus screw placement often results in excessive PCB costs.
As mentioned earlier, the screws in a wireless PCB have to be placed so that they don't interfere with the probe pattern. Traces are then added around the screw locations as part of a routing process during the design of the PCB. The placement of the screws is a manual operation and trace routing is significantly hindered by the existence of the screws. Both the manual intervention and trace routing difficulties caused by the screws during initial fixture fabrication, add significant costs of manufacture to the fixture.
In addition, the repair and maintenance of a conventional fixture is a large manual operation. Repairs often require taking the screws out of the wireless PCB and putting the screws back into the wireless PCB, which is a difficult and time-consuming operation. In addition, since it is a task that requires a substantial amount of human intervention, repairs can introduce more problems and ultimately result in test errors.
Making changes to a conventional fixture is a very costly enterprise. Engineering Change Orders (ECO's) are changes to the fixture, which are typically driven by design changes to the DUT. These ECO's often involve rewiring of the fixture, which might require changes to the wireless PCB. As a result, there is processing overhead with these changes as well as the opportunity for error with these changes. The screws once again need to be removed, re-inserted, and then uniformly tightened to balance any loads on the wireless PCB board during ECO's.
In addition, since companies often require changes in the DUT, there are continual manual changes to the fixture and the wireless PCB. Long fixture implementation times, to accommodate the changes in the DUT, result in a delay in manufacturing time, product delivery, increased costs, etc. In addition, the wireless PCB has to be properly aligned to provide for proper targeting and good contacts between the probe's and the wireless PCB during these changes. If good contacts are not established between the wireless PCB and the probes, it is very difficult to tell whether the DUT is failing or whether there is a misalignment or bad probe contact in the fixture.
In addition to the issues associated with configuring testers for each individual test, as each DUT is tested, the tester must be reconfigured to work with each new DUT. Furthermore, test capability extensions for test that cannot be implemented by an unmodified tester may be necessary on the same DUT. To perform test requiring capability extensions, conventional methods make use of custom fixture electronics. Custom fixture electronics include PCBs that are specifically designed to perform a specific test. Currently, implementing custom fixture electronics requires that a fixture builder design a custom PCB to provide the functionality for a specific DUT test. The custom PCB must then be installed in the test fixture, in a specific manner. Specific test may then be generated using the custom PCB. However, should the need arise to perform other test or to test another DUT, the custom PCB is practically useless. A new custom PCB must be designed and developed to perform new test or test on a different DUT.
Thus, there is a need in the art for a method and apparatus that minimizes screws in a wireless PCB. There is a need in the art for a fixture that is easily repaired. There is a need in the art for a fixture that can easily accommodate changes. There is a need in the art for a fixture that is easily assembled, debugged, and maintained. There is a need for a method and apparatus that facilitates custom DUT design and testing.